Improve testling coverage.

Fix a `Object.setPrototypeOf` test that was missing an `it` clause, leading
to confusing output on console (Opera 12, IE).

Add promises tests to `testling.html`; define `assert` in `test_helpers.js`.

Note that this reveals a number of issues with Chrome's built-in `Promise` implementation; a patch to shim in our implementation on Chrome will follow.